In the oilfield, equipment occasionally becomes stuck in a well. A stuck object or "fish" may either be part of a drilling string which became stuck during drilling of an oil well, or it may be production equipment being removed from an existing well bore during workover operation.
The oilfield jar is a tool used when either drilling or production equipment has become stuck to such a degree that pulling from the surface is not sufficient to dislodge the stuck components. The jar is placed in the drill string in the general region of the stuck object. It allows operation of the rig at the surface to cause an impact to be delivered to the wellbore string in the area of the stuck fish. The jar incorporates a pair of telescoping tubular parts which contain a time delay mechanism, so that the drill string can be stretched prior to the parts moving relative to each other. Each of the telescoping parts carries an impact surface, which are brought together rapidly once the drill string has been stretched and the parts are free to move relative to each other. This causes an impact or jar to occur which is transmitted to the stuck fish. Jars may be double acting units with a second pair of impact faces so that they may deliver a jar upwardly and a bump or jar downwardly, as is known in the art. Such a tool is referred to as a bumping and jarring tool.
In practice, there are two types of oilfield jars commonly used: the mechanical jar and the hydraulic jar. They differ in how the required time delay is achieved.
The mechanical jar comprises two telescoping parts and a mechanical latching system. The latching system restrains the parts from telescoping until a certain load is exceeded. For example, in a 63/4" mechanical drilling jar, the tripping load may be 60,000 lbs. force. Of course, in order to cause the latch to release, the operator must pull up on the drill string an amount equal to the weight of the drill string above the jar plus the tripping load of the jar. When the tripping load is applied, the mechanical latch releases and the members are rapidly extended with respect to each other, causing the impact faces to strike.
Mechanical jars are not versatile. The tripping load is preset at the surface and cannot be changed satisfactorily while the tool is downhole. Mechanical jars have been designed which require torque to be applied from the surface through the pipe to the tripping mechanism. This gives some variability as to the tripping load, but it can be dangerous to rig floor personnel and it is difficult to control the tripping load.
As well, mechanical jars have the disadvantage that wear of the latch components causes variation in the tripping point, and failures of the moving parts occur with regularity. The cost of manufacture and maintenance of mechanical jars is also high.
However, mechanical jars do afford the advantage that the jar can be rapidly fired and reset. Also, because they fire at a preset level, the impact delivered to the fish is known. Further, mechanical jars will not accidentally fire while tripping in or out of a hole.
The second type of oilfield jar commonly used is the hydraulic jar. The hydraulic jar has two telescoping parts with an internal liquid-holding space. Fluid is metered slowly from one chamber to another in the liquid-holding space when axial pull is applied to the tool. The rate at which the fluid is metered depends upon the load on the tool. Thus, a variation in impact can be attained at the discretion of the operator. Hydraulic jars are preferred in crooked holes, in which it is difficult to apply high tripping loads. Stated otherwise, if it is only possible to apply a low tensile pull to the tool due to contact of the drill string with the wellbore wall, the tool will still eventually fire (although with low impact).
Disadvantages of the hydraulic jar include that it may accidentally fire when being tripped in or out of a well. This can be dangerous to personnel. If excessive force is applied at a rapid rate, the hydraulic chamber wall can rupture. The jar cannot be fired as quickly as a mechanical jar. And finally, there is some uncertainty as to when it will fire and at what impact.
With this background in mind, it is the object of the present invention to provide a hybrid hydraulic/mechanical jarring and bumping tool that incorporates the dual capacities of being able:
to consistently and quickly fire at a pre-determined tensile load applied at the tool; and yet PA1 still fire (although with a longer time delay) if the pre-determined tensile load does not reach the tool. PA1 It is a hydraulically controlled and actuated jar that can fire at a pre-set mechanically-controlled tripping load; PA1 It preferably can independently and selectively jar up or bump down; PA1 It preferably can fire at or below the pre-set mechanically-controlled tripping load; and PA1 It is not subject to overloading.